Method of coating preformed ceramic cores

ABSTRACT

THIS INVENTION RELATES TO A METHOD OF COATING PREFORMED CERAMIC CORES WITH A THIN LAYER OF A REFRACTORY OXIDE, SAID OXIDE SERVING AS A REACTION BARRIER BETWEEN THE CERAMIC CORE ANDMOLTEN METALS, CONTAINING REACTIVE ALLOYING ELEMENTS. MORE SPECIFICALLY, THIS INVENTION RELATES TO A METHOD OF FORMING A CONTINUOUS IN SITU COATING ON A CERAMIC CORE BY IMMERSING SAID CORE IN A MOLTEN METAL BATH CONSISTING OF A NON-REACTIVE BASE OF SOLVENT METAL SUCH AS NICKEL OR COBALT, AND AT LEAST ONE SOLUTE METAL WHOSE FREE ENRGY OF OXIDE FORMATION IS LESS THAN MINUS 160 KILLOCALORIES PER MOLE OF OXYGEN AT 1260* C., SUCH AS ALUMINUM, ZIRCONIUM OR HAFNIUM.

United States Patent Oflice 3,824,113 Patented July 16, 1974 3,824,113 METHOD OF COATING PREFORMED CERAMIC CORES Ted A. Loxley, Mentor, Harold L. Wheaten, Kensington, and John M. Webb, Chagrin Falls, Ohio, assignors to Sherwood Refractories Inc., Cleveland, Ohio No Drawing. Filed May 8, 1972, Ser. No. 251,252 Int. Cl. B22c 3/00; B44d 1/20 U.S. Cl. 117--5.2 29 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of coating preformed ceramic cores with a thin layer of a refractory oxide, said oxide serving as a reaction barrier between the ceramic core and molten metals containing reactive alloying elements. More specifically, this invention relates to a method of forming a continuous in situ coating on a ceramic core by immersing said core in a molten metal bath consisting of a non-reactive base of a solvent metal, such as nickel or cobalt, and at least one solute metal whose free energy of oxide formation is less than minus 160 kilocalories per mole of oxygen at 1260 C., such as aluminum, zirconium or hafnium.

BACKGROUND OF THE INVENTION The widespread use of the gas turbine engine has provided much of the impetus to the development of improved high temperature materials and fabrication techniques. Engine performance is significantly improved as the operatng temperature, referred to as turbine inlet temperature, is increased. Materials to perform satisfactorily as structural components such as blades and vanes in the turbine section of the engine must possess high temperature creep-rupture strength, resistance to oxidation, stability, thermal fatigue resistance, as well as adequate ductility to prevent premature brittle failures. Turbine blades and vanes are commonly made from nickeland cobalt-base alloys since it is possible to achieve, through judicious alloying, the desired combination of high temperature properties. Unfortunately, alloy development is restricted in respect to increased strength at ever increasing temperatures by the melting range characteristic of the system and, in the case of nickeland cobalt-base alloys, this ranges from about 1200 to 1300 C. Thus, there is little beyond the currently available alloys, such as B 1900, MAR-M-ZOO, TRW 6A and MARM-509, in the nickeland cobalt-base systems since they are melting-point limitedthat is, they have useful high-temperature strengths up to about 1150* C. or very close to their solidus temperatures. Of course, there are other possible alloy systems, such as those based on the refractory metals, which have significantly higher melting ranges, but serious shortcomings, such as poor oxidation resistance, have precluded the use of these materials.

Faced with the lack of improved high-temperature materials, turbine designers have turned to intricate air cooling schemes in order to increase turbine inlet temperatures while maintaining metal operating temperatures within the operating capability of the alloy being used. In order to cool the structural components (i.e., the blades and vanes), it is necessary to have an internal cavity in the airfoil of the blade and vane which is designed to provide adequate cooling. In practice, these internal cavities are quite complex and may have fins, posts, slots, crossover holes, and the like. Some of these are present for structural consideration while others are necessary for maximum heat transfer.

The actual production of hardware with complex internal cavities necessitates the use of investment casting using a preformed ceramic core to form the cavity. Although this is primarily done for economic reasons, investment casting allows the use of higher strength alloys that could not be hot worked to shape.

The first step in the production of a hollow airfoil by investment casting is the placement of the preformed ceramic core in a cavity die followed by the injection of an expendable pattern material. around the core. The pattern material is usually wax but could also be frozen mercury, a synthetic plastic or a water-soluble material. Sprue components including gates, downpoles, and the like are also injected and, together with the cored patterns of the part desired, are assembled into clusters. The wax assembly is then dipped in a ceramic slurry, sanded with refractory grains, and dried, as disclosed, for example, in U.S. Pat. No. 3,196,506 or U.S. Pat. No. 2,932,864. These steps are repeated until there is formed a shell of suflicient thickness, generally about Ms inch. The mold is then dehumidified at a low relative humidity and the wax is removed by the application of heat. Autoclave and flash fire dewaxing are the most commonly used methods. After firing and cleaning, the mold is preheated to a specified temperature and molten metal is cast into it. In the case of blades and vanes for gas turbine applications, melting and casting is generally performed in a vacuum to obtain maximum properties of the alloy being cast. The mold and consequently the cores are exposed to temperatures ranging from about 800 to 1100 C. during the preheat cycle. Metal pouring temperatures are generally between about 1400 and 1650 C.

After the castings have cooled and have been cut off from the gates, the cores are removed by dissolving the ceramic core body. This operation is referred to as leaching, and the ability of a ceramic core to dissolve is called leachablity. The choice of a core material and a solvent to remove it are limited by the fact that virtually no attack is allowed on the metal casting. This is a severe limitation because most refractory oxdes are only soluble in strong solvents which would also attack the base metal. In addition, the preformed ceramic core must be capable of being fabricated into complex shapes by an economical technique, must have sulficient room temperature strength to resist the pressure during wax injection, must have adequate elevated temperature strength to withstand stresses due to non-uniform metal flow, must have dimensional stability during preheating and pouring, and must have sufiicient refractoriness or inertness to avoid reactions with the cast molten metal prior to solidification. Obviously, an optimization of properties is required to meet the needs listed above.

One of the most significant compromises which must be made is a result of the leachability requirement. Experience has shown that the only practical method of achieving core removal from nickeland cobalt-base alloy castings without deleteriously affecting the alloy is to use a high-silica core body (i.e., at least 40 percent by volume) whch can subsequently be removed in an alkali metal hydroxide, such as sodium or potassium hydroxide. Leaching is generally done in an autoclave using a 30 to 40 percent aqueous solution operating at a temperature from about to about 200 C.

Silica is an oxide which is thermodynamically unstable when in contact with alloys containing reactive elements, such as titanium, zirconium, hafnium, aluminum, and the like. Thus, the reaction Silica+Reactive Metal=Silicon+Reactive Metaloxide will occcur. The reactive metal oxide can extend into the metal casting leading to unacceptable quality. In addition.

silica can be reduced to SiO which, at the temperatures and pressures in airfoil investment casting, is a gaseous phase and will form gas pits in or near the core passage of the casting.

The high strength, high temperature nickeland cobaltbase alloys contain reactive alloying elements, such as aluminum, titanium, zirconium, and hafnium. For example, one commonly used nickel-base turbine alloy dcsignated PWA 1455 contains 6 percent aluminum, 1 percent titanmium, and about 1.25 to 1.5 percent hafnium (by weight), which are necessary additions to develop the high temperature strength and ductility characteristic of the material. When this alloy is cast around a preformed high-silica ceramic core, the reactive metals therein can react with the silica in the manner described above to produce surface defects which seriously reduce the quality of the casting and may render it unacceptable. The properties of the metal may also be deleteriously affected near the surface by depletion of the hafnium and/ or the titanium.

Destructive microexamination of turbine-blade castings made from superalloys, such as PWA 1455, MAR- M509 and the like, shows that surface portions of the casting formed adjacent the silica core have carbide particles which are frequently oxidized to a depth of 0.3 mil or more. Where such internal carbide oxidation extends 0.4 mil or more from the internal surface of a rotating gas turbine engine part, such as a turbine blade, the casting is rejected as not being within tolerable limits. As a result, a substantial percentage of castings are scrapped.

Reactive metals, such as hafnium and zirconium, can be a serious problem even when the percentages are relatively small, such as 0.5 to 2 percent or even less. They can react with the silica or silicates in the ceramic core during solidification of the molten metal.

Carbon is also present in the superalloys commonly used for turbine blades and is used in amounts of about 0.4 to about 1.0 percent by weight in typical cobalt-based superalloys. Since carbon can react with silica, it also presents a problem at the metal-core interface and reduces the quality of the casting surface.

A common cobalt-based superalloy, such as MAR-M- 509, may, for example, contain about 0.6 percent carbon, about 0.2 percent titanium, and about 0.5 percent zirconium (by weight). An alloy of this type contains large amounts of less reactive metals, such as about 55 percent cobalt, about 23 percent chromium, about percent nickel, about 7 percent tungsten, and about 3.5 percent tantalum (by weight). Metals, such as chromium, tungsten, molybdenum, tantalum, and vanadium, are employed to promote the formation of the carbides from which the cobalt-based alloys derive their strength. However, these carbides are subject to oxidation due to the presence of the elements which react with silica during the casting process, and the internal carbide oxidation can sometimes be substantially greater than the maximum tolerable depth limit of 0.4 mil.

The rate of oxidation of the reactive metals of the superalloy is reduced somewhat by including in the silica core substantial amounts of refractory oxides, such as alumina or zirconia, but such amounts are limited because of the leaching problem and should be less than 30 percent. In turbine blade cores, it is preferred to use at least 95 percent silica to facilitate leaching. Prior to the present invention, there was no practical way to provide a leachable ceramic core which did not react with the hafnium, titanium, zirconium, carbon or other reactive elements of the superalloys used for gas turbine engine castings, and the industry continued to make such castings using conventional high-silica cores.

Because of the difficulties in meeting all the different requirements for a satisfactory ceramic core, it was not known how to form an effective and acceptable protective coating on a ceramic core. Satisfactory results could not be obtained by applying colloidal alumina to the surface of a core or by applying an aluminum nitrate solution in an attempt to obtain an aluminum oxide film.

Various techniques have been used in other fields to deposit oxide coatings on an article such as vapor deposition, flame spraying, plasma spraying and the like, but these cannot be used on ceramic cores for turbine airfoils or the like. The latter cores involve many critical limitations and do not permit substantial changes in the formulation or structure because of extreme fragility, geometric complexity, dimensional considerations and other problems. For example, vapor deposition techniques are unacceptable because the gaseous phase penetrates the permeable core body and costs the interior as well as the exterior of the core. This can lead to cracking and distortion as Well as gross chemical changes in the core body with the result that the core does not have the desired leachability characteristics and other necessary properties.

DESCRIPTION OF THE INVENTION This invention relates to a method of forming a thin impervious reaction barrier of a stable refractory oxide on the surface of a preformed ceramic core. More specifically, it relates to a method of providing a non-reactive preformed ceramic core for the investment casting of high temperature alloys containing reactive alloying elements.

The present invention involves the discovery of an improved leachable high-silica ceramic core for turbine blades and other gas turbine engine parts which has all of the essential properties for such use and which reduces or minimizes the internal surface discontinuities and internal carbide oxidation problems associated with use of the common superalloys. It has been found that a thin protective refractory oxide coating can be formed on the surface of a fragile complex preformed ceramic core by immersing the core in a molten metal bath containing a non-reactive or relatively inactive solvent metal, preferably nickel or cobalt, and at least one reactive metal whose free energy of oxide formation is less than minus kilocalories per mole of oxygen at 1260 C., preferably aluminum, hafnium, zirconium or magnesium. The bath usually contains a combination of three or more metals to provide an alloy with suitable melting characteristics to permit treatment of the silica core for the desired period of time under a non-oxidizing atmosphere at a temperature in the range of from about 1100' C. to about 1500 C. or 1600 C. and to obtain the desired thickness of oxide coating. Such thickness is preferably about 0.05 to 0.4 mil for most cases but may, in some instances, be as high as 1.0 mil. The reactive metals should be used in minor amounts by weight to limit the rate of reaction at the temperature of the molten bath which is at least 1000 C., and, therefore, the amount of the reactive metals in the molten bath is preferably no more than 20 atomic percent and usually no more than 10 atomic percent.

An object of the present invention is to improve the internal surface characteristics of gas turbine airfoils or the like so that they can withstand more severe operating conditions and have a greater useful life.

A further object of the invention is to provide a leachable high-silica core for gas turbine engine parts which minimizes surface defects and internal carbide oxidation during casting of superalloys containing substantial amounts of reactive elements.

A still further object is to provide a simple, reliable, economical process for producing a uniform continuous refractory oxide coating on a complicated turbine airfoil core containing large amounts of silica.

Another object of the invention is to avoid depletion of essential reactive metals near the internal surface of a hollow turbine airfoil casting.

These and other objects, uses and advantages of the invention will become apparent to those skilled in the art from the description and claims which follow.

The present invention is concerned with leachable preformed ceramic cores containing at least 30 or 40 percent by volume and usually a major portion by volume of silica. The invention is applicable to ceramic cores for gas turbine engine parts, including turbine vanes, turbine blades, jet diffuser housings, afterburner flaps, and the like, particularly those subjected to hot combustion gases. The cores are frequently of complex shape wth extremely thin cross sections or configurations making leaching more difiicult. It is preferred to employ a core containing 70 to 100 percent by volume of silica. Where leachability is extremely important it is often preferable to employ a core containing at least 90 to 95 percent by volume of silica.

The silica used in the core is preferably fused silica and usually has a particle size not in excess of 120 microns and an average particle size in the range of to 60 microns, but the size of the particles may vary considerably. A small or minor porportion of the silica may be in crystalline form. Where high density is desired the core may contain preselected mixtures of silica of different sizes, the maximum usually being less than 130 microns. A typical silica core may contain particles varying in size from 10 to 90 microns with an average particle size of perhaps to 50 microns.

The core may contain 70 to 99 percent by volume of silica and 1 to percent by volume of another refractory oxide, such as zircon, zirconia or alumina. The other refractory oxide usually has a particle size not in excess of about 100 microns which may be in the same general range as that for the silica particles. The refractory oxide preferred for addition to the core is zircon which is usually used in a particle size of about 0.1 to 70 microns. It is common to use 70 to 80 percent by weight of silica and 20 to 30 percent by weight of zircon or zirconia to facilitate X-ray detection of residual core after leaching, but this makes leaching more difiicult because of the sludge problem.

One type of core which can be used in the practice of this invention is disclosed in U.S. Pat. No. 3,222,435 and employs an ethyl silicate binder with a catalyst such as magnesium oxide or aluminum oxide. Such a binder preferably comprises, by weight, about 30 to 60 percent ethyl silicate, about 30 to 60 percent alcohol, about 0.05 to 0.5 percent concentrated hydrochloric acid, and about 1 to 10 percent water. One part by weight of such binder may be mixed with about 2.5 to 4.5 parts by weight of the dry refractory mix.

For example, as disclosed in said patent, 5300 milliliters of ethyl silicate can be mixed with 4200 milliliters of ethyl alcohol and 500 milliliters of one percent hydrochloric acid. One part by weight of the resulting liquid carrier can then be mixed with three parts by weight of a dry mix consisting of 60 percent by weight of fused silica, 39.5 percent by weight of zirconium orthosilicate (zircon), and 0.5 percent of magnesium oxide. The zirconium silicate of the mix is in the form of a powder with a particle size of at least 0.1 micron which will pass through a ZOO-mesh (U.S. Standard Sieve) screen. The fused silica of the mix comprises 40 percent by weight of coarse particles which pass through a 60- mesh screen and are retained on a 100-mesh screen, and 60 percent by Weight of finer particles which pass through a IOU-mesh screen but are retained on a 300-mesh screen.

Ten milliliters of a 25-percent solution of ammonium acetate in water may be added per 1000 milliliters of said liquid carrier to provide an accelerator for the ceramic slurry.

The ceramic slurry described above may be used to mold a ceramic core as disclosed in said Pat. No. 3,222,435 and such core may then be torched or heated in a furnace to a temperature of 900 to 1200 C. to burn ofi combustibles before the core is dipped in a molten metal bath according to the process of the present invention.

The invention may be applied to many different silica cores formed from various compositions by various methods. The cores can be shaped by injection molding, by extrusion, by pressing, by slip-casting and various other methods. They can be mass produced by pouring a ceramic slurry into plastic gravity-type molds, for example.

A substantial number of different core compositions can be used provided there is the required amount of silica present. The core composition can employ various lowtemperature and high-temperature binders including shellac and other organic compounds, silicone resins, colloidal silica, colloidal alumina, colloidal zirconia, sodium silicate, ethyl silicate or the like or combinations thereof as disclosed, for example, in U.S. Pats. Nos. 3,307,232, 3,450,- 672 and 3,222,435. The organic compounds or other combustibles should be removed by heating the core to a high temperature after the core is removed from the core mold and before immersing the core in a molten metal bath according to this invention, for example, by heating the core to a temperature of 1000 to 1200 C.

Preformed ceramic cores treated by the method of this invention may be used for investment casting in the usual manner. The core is covered by a destructible pattern, which may be a wax, a synthetic resin or frozen mercury,- and a shell mold is formed around the pattern in a suitable manner, for example as disclosed in U.S. Pat. No. 3,196,506; 2,932,864; 3,171,174; 3,307,232 or 3,452,804.

Today, most turbine engine airfoils are made by the lost-wax process which usually involves autoclave or flash fire dewaxing before the molten metal is poured into the shell mold, for example at temperatures between 800 and 1200 C. It is common to preheat the mold and consequently the core to a temperature of l000 to 1200 C. just before the casting operation. The metal is commonly poured at a temperature from 1400 to 1550 C. and, in some cases, the mold is preheated to a temperature of 1400 to 1500 C. which is high enough to cause devitrification. The quality of the core must be very high to maintain its dimensions and otherwise perform satisfactorily under such extreme conditions.

Core made according to the present invention are useful when casting alloys containing significant amounts of elements which react with silica or silicates at the temperatures encountered during casting, particularly alloys containing chemically significant amounts of elements such as titanium, zirconium, hafnium, aluminum, yttrium, cerium or carbon. The invention is particularly concerned with preformed high-silica cores used in the casting of cobalt-based and nickel-based superalloys con taining amounts of titanium, zirconium, hafnium, yttrium, cerium or similar reactive metals sulficient to cause substantial oxidation at the core-metal interface. For example, when casting in shell molds containing high-silica cores, superalloys containing a total of only 0.3 to 2 percent of zirconium and/or hafnium can cause serious problems that can be avoided by treating the cores according to the process of this invention.

In order to obtain the desired refractory oxide coating on the preformed ceramic core, the core is immersed in a molten metal bath containing one or more reactive elements having free energies of oxide formation less than minus kilocalories per mole of oxygen at 1260 C. which react with silica in the desired manner to produce a stable refractory oxide layer. The reactive elements suitable for this purpose include Group He: metals, such as beryllium, magnesium or calcium; Group IIIa metals, such as aluminum, yttrium or lanthanum; Group IVa metals, such as titanium, zirconium, hafnium or thorium; and cerium and the other rare earth metals. The periodic classification used herein and in the claims is in accordance with that used in Inorganic Chemistry by Fritz Ephraim,

published 1949 by Interscience Publishers, -Inc. While some advantages of the invention are obtained using titanium in the molten metal bath, it is preferred to use a metal whose free energy of oxide formation is less than minus 165 kilocalories per mole of oxygen at 1260 C. and substantially less than that of silicon. In general, the preferred metals for treating the silica core are zirconium, hafnium, aluminum and magnesium. However, good results can also be obtained using the rare earth metals, particularly cerium, yttrium and lanthanum. It may be less practical to use many of the other reactive metals mentioned above because of cost and other factors. For example, beryllium is toxic, and thorium is radioactive. Calcium oxide hydrates in the presence of moisture. While a metal may have certain disadvantages, such as those mentioned above, it may nevertheless be desirable when alloyed with other metals.

In carrying out the process of the present invention, there are a number of variables to be considered including the temperature of the molten metal bath, the immersion time, the type and level of the reactive alloying element or elements, the chemistry of the core, and the desired thickness of the oxide coating. The bath temperature must be high enough to achieve the desired reaction of the metal with silica in a reasonable period of time and, therefore, is preferably at least 800 C. and usually at least 1000 C. A relatively high bath temperature may be desirable to shorten the immersion time and increase the rate of oxide formation, but too high a batch temperature and/ or too long an immersion time can lead to additional sintering and shrinkage or distortion of the core. Some of the very fragile or geometrically complex cores may be damaged in a short period of time at a temperature of 1400 C. Some may be satisfactory after being subjected to a temperature of 1500 to 1550 C. or higher for a short period of time, such as to seconds, sufficient to provide a protective oxide coating of adequate thickness. The maximum temperature and maximum immersion time depend both on the shape and the composition of the core. Generally, it is preferred to carry out the process of this invention with a molten metal bath at a temperature of about 1100 C. to about 1550 or 1600 C.

In order to achieve the desired bath temperature, it is usually essential to use at least 2 or 3 different metals to produce an alloy of the proper melting point or liquidus temperature (for example, below 1500 C.). Lower liquidus temperatures allow greater flexibility in the choice of bath temperature. Also the alloy used in the bath should contain the proper amount of the reactive alloying element or elements so that the reaction with the silica core proceeds at a suitable rate. In order to obtain an alloy having the proper reactivity and liquidus temperature, the reactive metal can be mixed with various non-reactive or slowly reactive elements. For example, the liquidus temperature of a nickel-hafnium alloy containing 2 atomic percent hafnium is only slightly lower than the melting point of pure nickel (about 1450 C.) but this can be lowered to about 960 C. by adding around 29 percent by weight of silicon. When practicing the present invention, other elements such as chromium, manganese, boron and the like can be used, but it is preferable to avoid elements which are normally considered tramp elements in the high temperature nickeland cobalt-base alloys. Therefore, it is preferable to minimize or avoid use of many of the low melting point elements, such as lead, bismuth, zinc, tin and the like in the practice of this invention.

The molten metal bath used in the process of this invention usually contains substantial amounts of one or more non-reactive solvent metals, such as copper, gold, silver, silicon, germanium, iron, cobalt or nickel. Cobalt and nickel are usually preferred. Less common metals, such as platinum and the other metals of the platinum group, may also be used. It is preferable not to use gold or silver.

Good results can be obtained where the metal bath consists of nickel-based or cobalt-based alloys similar to those used for casting turbine blades and other high quality metal products. For example, the bath may employ a nickel-based alloy containing aluminum, chromium, tungsten and/ or molybdenum and also containing small amounts of one or more reactive metals, such as zirconium or hafnium. The alloy used in the bath could also be a cobalt-based alloy containing chromium, tungsten, molybdenum, tantalum and/or vanadium. In some cases the bath could comprise a conventional alloy, such as MARM- 509, B 1900, MAR-'M-200, TRW 6A, or Monel metal, or one or more of such alloys enriched with additional amounts of one or more reactive metals, such as zirconium or hafnium. For example, the alloy could contain 3 to 8 percent by weight or more of such reactive metals or enough to cause reaction with the silica at a rate significantly higher than that which would be obtained with the original alloy at the same temperature.

A large number of different nickel-based alloys suitable for casting modern turbine blades and the like could be used in the molten metal bath. A commonly used alloy, such as B 1900, contains (by weight) about 0.1 percent carbon, about 8 percent chromium, about 6 percent molybdenum, about 1 percent titanium, about 6 percent aluminum, about 10 percent cobalt, about .08 percent zirconium, about .015 percent boron, about 4 percent tantalum, and the balance (over 62 percent) nickel. This alloy could readily be modified for use as the molten metal bath in the process of the present invention, for example, by adding 1 to 5 percent by weight of one or more reactive metals such as zirconium, hafnium, cerium or the like. This modification also applies to PWA 1455, which is the same as B 1900 but contains 1.25 to 1.5 percent by weight of hafnium.

While the metal alloy used in the molten bath according to this invention can contain 3 to 10 or more different elements commonly found in precision metal alloys, it is usually preferable to minimize or eliminate elements not needed to obtain the desired liquidus temperature, particularly those which would detrimentally affect the core if they were picked up in trace quantities. The amount of carbon is usually in the range of from about 0.02 to 1.0 percent by weight but can be substantially less. The total amount of such elements as lead, bismuth, zinc, tin or the like having melting points below 450 C. is preferably minimized (for example, below 1 percent and preferably below 0.01 percent by weight).

A major amount by weight of the alloy constituting the molten metal bath usually comprises solvent metals or other metals which do not react readily. Thus, 70 to 98 percent by weight or more of the metal bath may be one or more metals, such as copper, germanium, silicon, vanadium, columbium, tantalum, chromium, molybdenum, tungsten, iron, cobalt or nickel. No more than a minor amount by weight of the reactive metal is needed in the bath, and larger amounts are not wanted because the rate of reaction with silica would then be excessive at the high temperatures required to melt the alloy. The percentage of reactive metal can be higher for a metal, such as aluminum, than for a Group IVa metal, such as hafnium. In general, the total amount of the metals reactive with the silica should not exceed 20 atomic percent and preferably is in the range of about 0.5 to about 10 atomic percent. The minimum may be substantially less (for example, 0.2 to 0.3 atomic percent) where highly reactive metals, such as zirconium or hafnium, are used at bath temperatures of 1400 to 1500 C. or higher. However, the amount of the reactive metal to be effective should be sufficient to produce a refractory oxide coating on the core with a thickness greater than 0.01 mi] and adequate to protect against excessive oxidation of a metal casting formed on such core.

The amount of the reactive metal is inversely related to its reactivity and also to the bath temperature employed. While it may be satisfactory to employ more than 1O atomic percent of a metal, such as aluminum, it may be preferable to use only 1 to atomic percent of a more reactive metal, such as hafnium or zirconium. If a lower bath temperature is employed, it may be possible to employ a much greater amount of the reactive metal.

The immersion time employed in the process of this invention can vary substantially and can be several hours at 1100 C. or 5 to 30 seconds at 1450 to 1500" C. Generally, it is impractical to immerse the silica cores in the molten bath for more than 2 hours, and the immersion time at 1l00 to 1200 C. would be preferably no more than 1 hour and could be only a few minutes or possibly less than one minute with highly reactive metals. The immersion time at 1400 to 1500 C. would, of course, be less, and should probably be in the range of 5 to 30 seconds for most turbine-blade cores. Where the maximum permissible immersion time is short due to the high bath temperature or the delicate nature of the core, the reactivity of the metal with the silica must be high enough to provide a stable refractory oxide coating of adequate thickness on the surface of the core within that limited immersion time, and it is, therefore, desirable to use highly reactive metals, such as hafnium, zirconium or the like. The amounts of such metals may be relatively small, such as 0.5 to 3 percent by weight, particularly where the core contains 80 percent or more by Weight of silica.

The refractory oxide coating formed on the core is preferably substantially free of cracks and fissures and thick enough to prevent diffusion of the reactive metal through the coating to the silica of the core. Because immersion times are limited and the expansion characteristics of the oxide coating are different from those of the silica core, a thick oxide layer is generally undesirable. The maximum thickness depends on various factors, such as the size of the core, the shape of the core, the method of core removal, the importance of dimensional accuracy, etc. In general, the average thickness of the oxide coating should be no greater than 1 mil and preferably no greater than 0.5 mil. For most preformed ceramic cores for gas turbine engine parts, the preferred average thickness of the oxide coating formed by the process of this invention will be about 0.05 to 0.4 mil. In some instances, some advantages of the invention may be obtained with a coating thickness as low as 0.01 mil, but thicker coatings provide better protection against the metal-silica reaction. The oxide coating on the silica core should cover the silica particles and be thick enough to materially improve the resistance to internal carbide oxidation when the core is used for casting conventional nickel-based and cobaltbased superalloys, such as MARM509. The coating is significantly effective if it will reduce the average depth of the internal carbide oxidation by an average of 20 percent or more as compared to the same core without the coating as determined, for example, by destructive microexamination. A relatively thin coating can sometimes reduce such depth by 70 to 90 percent or more. This makes it possible to cast alloys containing much higher percentages of highly reactive metals, such as zirconium or hafnium, without excessive surface degradation.

Because the oxide barrier layer on the core is formed by reaction of the molten metal with silica, other refractory particles at the core surface, such as particles of aluminum oxide or zirconium oxide, may produce small pinholes in the barrier layer. The layer is effective and can be considered substantially continuous in spite of the presence of such holes. Where the core contains at least 70 percent by volume of silica, the barrier layer will be substantially continuous even when it has a very small thickness. Where the volume percent of silica is 90 to 95 percent or higher, the barrier layer can be substantially imperforate or substantially free of such holes.

In carrying out the process of this invention, the molten bath is formed by melting the alloy containing a reactive solute metal, such as zirconium, hafnium, or aluminum,

and at least one solvent metal, such as copper, nickel, cobalt or iron. The bath is held at a suitable temperature, such as 1100 to 1500 C., and the preformed high-silica cores are immersed for a predetermined period of time, such as 10 seconds to 1 hour, which is inversely related to the temperature and to the reactivity of the metal.

The oxide coating should be formed on the core under a non-oxidizing or reducing atmosphere or in a vacuum. Excellent results can be obtained by carrying out the process in an atmosphere of an inert gas, such as argon or helium.

The alloy used in the molten bath may, for example, be a nickel-based alloy containing 55 to percent by weight of nickel and up to 5 percent by weight of a reactive metal, such as zirconium and/or hafnium. Such alloy may, for example, be similar to Monel metal and contain 65 to 70 percent of nickel and 25 to 30 percent by weight of copper. A copper-base or iron-base alloy with a liquidus temperature below 1500 C. could also be used, but a nickel-base alloy would be preferred.

The alloy used in the molten bath often contains a major amount by weight of cobalt and/or nickel and a minor amount of one or more other metals which do not react readily with silica. For example, the alloy may be a nickel-base alloy having a liquidus temperature from 1000 to 1500" C. and containing a major portion by weight of nickel, 0 to 30 percent by weight of cobalt, 0 to 30 percent by weight of copper, 0 to 40 (usually no more than 30) percent by weight of silicon, 0 to 30 percent by weight of iron, and from 1 to 10 percent by weight of one or more of the aforesaid reactive metals, such as aluminum, zirconium or hafnium. The alloy may also be a cobalt-base alloy containing a major portion by weight of cobalt, 0 to 30 percent by weight of nickel, O to 30 percent by weight of silicon, 0 to 30 percent by weight of copper, 0 to 30 percent by weight of iron, and from 1 to 10 percent by weight of said reactive metals.

The total amount of elements, such as lead, bismuth, zinc, tin, gold and silver, is relatively low (e.g., less than 5 percent) and is preferably minimized when the core is to be used for casting turbine engine parts.

The metal alloys used in the process of this invention can employ substantial amounts of other metals. For example, the alloys can contain 10 percent by weight or more of one or more metals, such as chromium, molybdenum, tungsten, vanadium, columbium and tantalum. Small amounts of additives or catalysts may also be used to promote formation of the desired barrier layer on the core.

The process of this invention is suitable for treating a typical turbine-blade core, for example, a core as shown in FIG. 7 of Pat. No. 3,356,130 having a length of 3 to 5 inches, a width of 1 to 3 inches and a thickness of 0.2 to 0.4 inch at the leading edge which gradually tapers down to less than 0.04 inch at the trailing edge.

Such a turbine blade core could, for example, be made from the illustrative composition of US. Pat. 3,222,435 which was previously described and which contained 60 percent by weight of fused silica, 39.5 percent by weight of zircon, and 0.5 percent by weight of magnesium oxide. After molding the core from such a composition and removing it from the mold, the core can be heated to a temperature of 1000 to 1100 C. long enough to burn off the combustibles.

Thereafter, such core can be immersed in a molten metal bath containing a suitable nickel-base alloy with a liquidus temperature below 1500 C., such as B 1900, plus an additional 3 to 4 percent by weight of hafnium or zirconium. Such bath may be located in a vacuum or in a closed furnace containing an atmosphere consisting essentially of argon or helium gas and may be maintained at a temperature above the liquidus temperature, such as 1400 to 1450 C, The above-described core may be immersed in this bath for a short period of time, such as 15 to 30 seconds, to form a refractory oxide coating com- -1 1 prising hafnium oxide and minor amounts of zirconium oxide and titanium oxide and having a thicknes preferably in excess of 0.03 mil and not in excess of 0.1 mil.

A turbine-blade core treated in this manner may be used in the conventional lost wax process during formation of a shell mold which is thereafter dewaxed and used for casting a nickel-base or cobalt-base superalloy, such as PWA 1455 or MARM509. The refractory oxide coating can greatly reduce internal carbide oxidation at the surface of the metal casting and may reduce the depth of such oxidation to less than half that which results when the same core is not coated by the method of this invention or even lower (for example, reduce the depth to less than 0.1 mil in the average casting).

The depth of internal carbide oxidation can be determined by destructive microexamination of the turbineblade casting and is the maximum which is observed during inspection of that casting. The term average depth as applied to such carbide oxidation is the average of the depths observed on a number of identical castings rather than the average depth on one casting.

While the refractory oxide barrier layer formed on the silica core by the process of this invention is usually formed by a single immersion in the molten metal bath, it will be understood that two or more immersions can be employed to increase the thickness and/or effectiveness of the barrier layer. The second or subsequent immersion can be in a different molten metal bath containing different reactive metals.

For example, the first immersion can be in a molten bath containing substantial amounts (for example, 1 to atomic percent) of a Group IIIa metal, such as aluminum, and the second immersion can be in a molten bath containing substantial amounts (for example, 0.4 to 20 atomic percent) of a Group IVa metal as defined above, such as zirconium or hafnium. A two-dip or multiple-dip process of this type would make it possible to use greater amounts of the reactive metals and/or higher temperatures in the second molten metal bath than would be prefererd in the first metal bath. It could also decrease the minimum time needed in the first bath so as to permit a higher temperature in the first bath than would otherwise be suitable.

As used herein, the term reactive metal refers to metals which are reactive with silica at high temperatures, such as 1000 C. and above. Unless the context shows otherwise, the amounts or percentages of the reactive elements disclosed herein relates to the amounts or proportion used in the molten metal bath in which the silica core is immersed or the amounts or proportions used in the alloy which is melted to form such bath.

Except as otherwise indicated, the high-silica cores which are to be immersed in the molten bath according to this invention contain at least 40 percent by volume of silica after being heated or fired in the conventional manner at a high temperature, such as about 900 to 1200 C. to remove combustibles.

It will be understood that the above description is by way of illustration, rather than limitation, and that, in accordance with the provisions of the patent statutes, variations and modifications of the specific methods and products disclosed in the specification and claims of this application may be made without departing from the spirit of the present invention.

Having described our invention, we claim:

1. A process of forming a stable refractory oxide reaction barrier on a preformed ceramic core for casting of gas turbine engine parts, said core containing at least 40 percent by volume of silica, said process comprising covering the surface of the core with a molten metal alloy having a liquidus temperature not in excess of about 1500 C. and containing at least one reactive metal which reacts with silica at the temperature of the molten metal to form a stable refractory oxide and containing at least one metal which does not react readily at said temperature, and causing said reactive metal to react with the silica on outer surface portions of said core for a period of time sufficient to form an effective oxide reaction barrier with a thickness not in excess of 1 mil, the free energy of oxide formation of said reactive metal being less than minus kilocalories per mole of oxygen at 1260 C.

2. A process according to claim 1 in which said metal alloy has a liquidus temperature from about 600 to about 1500 C. and contains about .05 to 20 atomic percent of said reactive metal.

3. A process according to claim 2 in which the alloy contains one or more reactive metals selected from the group consisting of beryllium, magnesium, calcium, aluminum, yttrium, lanthanum, cerium, titanium, zirconium, hafnium and thorium.

4. A process according to claim 2 in which the re active metal is one or more reactive metals selected from the group consisting of magnesium, aluminum, yttrium, zirconium and hafnium.

5. A process according to claim 4 in which the alloy contains no more than 10 atomic percent of said reactive metals.

6. A process according to claim 1 in which the alloy contains at least 60 atomic percent of one or more metals selected from the group consisting of copper, silicon, germanium, iron, cobalt and nickel.

7. A process according to claim 6 in which the alloy contains more than 0.1 atomic percent of one or more reactive rare earth metals whose free energies of oxide formation are less than minus kilocalories per mole of oxygen at 1260 C.

8. A process according to claim 6 in which the alloy contains a major portion by weight of nickel.

9. A process according to claim 6 in which the alloy contains a major portion by weight of cobalt.

10. A process according to claim 6 in which the alloy contains from about 0.2 to about 10 atomic percent of one or more Group IVa metals whose free energies of oxide formation are less than minus 165 kilocalories per mole of oxygen at 1260 C.

11. A process according to claim 6 in which the alloy contains from about 1 to about 10 atomic percent of one or more Group IIIa metals.

12. A process according to claim 1 in which said alloy contains a major portion by weight of nickel, up to 30 percent by weight of cobalt,'up to 30 percent by weight of copper, up to 40 percent by weight of silicon, and up to 30 percent by weight of iron, and from about 1 to about 10 percent by weight of one or more reactive metals and has a liquidus temperature from about 1000 to about 1500 C.

13. A process according to claim 1 in which said alloy contains a major portion by weight of cobalt, up to 30 percent by weight of nickel, up to 30 percent by weight of silicon, up to 30 percent by weight of copper, up to 30 percent by weight of iron, and from about 1 to about 10 percent by weight of one or more reactive solute metals and has a liquidus temperature from about 1000 to about 1500 C.

14. A process according to claim 1 in which said alloy contains at least 5 atomic percent of aluminum.

15.A process according to claim 1 in which said alloy contains a total of no more than 5 percent by weight of metals selected from the group consisting of lead, bismuth, zinc and tin.

16. A process according to claim 15 in which said alloy contains at least 10 percent by weight of one or more metals selected from the group consisting of chromium, molybdenum, tungsten, vanadium, columbium and tantalum.

17. A process according to claim 1 in which said alloy is heated to provide a molten metal bath having a temperature of 1100 to 1600 C. and said core is immersed in the bath for a period of time greater than 10 seconds and sufiicient to provide the core with said oxide reaction barrier.

18. A process according to claim 17 in which said core is immersed in said molten metal bath for a period of from about seconds to about 30 minutes which period is inversely related to the temperature and to the reactivity of the reactive metal.

19. A process according to claim 17 in which said core is immersed in said molten metal bath for a period of from about 10 seconds to about 1 minute while the temperature of said bath is from about 1250 to about 1600 C.

20. A process according to claim 19 in which said alloy contains from about 0.2 to about 10 atomic percent of one or more Group IVa metals.

21. A process according to claim 17 in which said core is immersed in said molten metal bath for a period of from about 1 minute to about 30 minutes while the temperature of said bath is from about 1100 to about 1200 C.

22. A process according to claim 17 in which said alloy contains from about 1 to about 20 atomic percent of one or more Group IIIa metals.

23. A process according to claim 17 wherein said core is immersed in said molten metal bath in a non-oxidizing environment for a period of from about 10 seconds to about 30 minutes which is inversely related to the bath temperature and sufiicient to provide the core with a continuous refractory oxide coating having a thickness of from about .05 to about 0.5 mil.

24. A process of forming a stable refractory oxide reaction barrier on a preformed ceramic core for casting of gas turbine engine parts, said core containing at least 40 percent by volume of silica, said process comprising covering the surface of the core with a molten metal having a liquidus temperature not in excess of 1500 C. and containing at least one reactive metal which reacts with silica at the temperature of the molten metal to form a stable refractory oxide, and causing the reactive metal to react wtih the silica on outer surface portions of said core for a period of time sufiicient to form an oxide reaction barrier with a thickness from about .05 to about 0.5 mil, the amount of said reactive metal being suflicient to form said barrier with said thickness, the free energy of oxide formation of said reactive metal being less than minus 160 kilocalories per mole of oxygen at 1260 C.

25. In a process for making a core for use in the investment casting of turbine engine parts, said core con taining at least 70 percent by volume of silica, the steps which comprise immersing said core in a molten metal bath having a liquidus temperature not in excess of 1500 C. and containing up to 20 atomic percent of one or more reactive metals which react with silica to form a stable refractory oxide layer, the free energies of oxide formation of said metals being less than minus 160 kilocalories per mole of oxygen at 1260 C., and causing said reactive metals to react with the silica at the surface of said core by heating to a temperature of from about 1100 C. to about 1550 C. for a period of time long enough to form a substantially continuous effective metal oxide barrier layer with a thickness greater than 0.01 mil and sufficient to prevent excessive carbide oxidation and gas formation when casting a cobalt-base alloy containing reactive metals, such as Group IVa metals.

26. In a process for making a core for use in the investment casting of turbine engine parts, said core containing a major portion by weight of silica, the steps which comprise immersing said core in a molten metal bath having a liquidus temperature not in excess of 1500 C. and containing up to 20 atomic percent of one or more reactive metals which react with silica to form a stable refractory oxide layer, the free energies of oxide formation of said metals being less than minus kilocalories per mole of oxygen at 1260 C., and causing said metals to react with the silica at the surface of said core by heating to a temperature of from about 1100 to about 1550" C. for a period of time long enough to form a substantially continuous effective metal oxide barrier layer with a thickness greater than 0.01 mil and sufficient to prevent excessive carbide oxidation and gas formation when casting a nickel-base alloy containing reactive metals, such as Group IVa metals.

27. A mold core for forming a hollow turbine air foil comprising a shaped body of a predetermined size and shape containing at least 70 percent by volume of silica and a stable effective metal oxide barrier layer with a thickness from about 0.01 to about 0.4 mil, said barrier layer being formed in situ by contacting the core with a molten metal alloy containing one or more metals whose free energies of oxide formation at 1260" C. are less than minus kilocalories per mole of oxygen, and which react with silica to form said barrier layer with said thickness.

28. A turbine-airfoil mold core according to claim 27 having a barrier layer with a thickness of from about 0.05 to about 0.2 mil formed from one or more metals selected from the group consisting of magnesium, aluminum, yttrium, zirconium and hafnium.

29. A turbine-airfoil mold core according to claim 27 wherein said barrier layer consists essentially of an oxide of a rare earth metal.

& Co., Chicago, Ill.

WILLIAM D. MARTIN, Primary Examiner I. H. NEWSOME, Assistant Examiner US. Cl. X.R.

1l7--114 R, 118, 123 A; l6472 

